Nonaqueous electrolyte and nonaqueous-electrolyte battery

ABSTRACT

The object is to provide a nonaqueous-electrolyte battery having high charge/discharge efficiency and excellent high-rate performance. This subject is accomplished by using a nonaqueous electrolyte which comprises an organic solvent and a lithium salt dissolved therein and is characterized by containing at least one quaternary ammonium salt in an amount of 0.06 mol/L or larger and 0.5 mol/L or smaller. This effect is thought to be attributable to the following mechanism: in a relatively early stage (stage in which the negative-electrode potential is relatively noble) in a first charge step, a satisfactory protective coating film is formed on the negative electrode by the action of the quaternary ammonium salt and, hence, the organic solvent employed in the nonaqueous electrolyte is inhibited from decomposing.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte and anonaqueous-electrolyte battery. More particularly, the invention relatesto an improvement in nonaqueous electrolytes.

BACKGROUND ART

Nonaqueous-electrolyte batteries, in particular, lithium secondarybatteries, are recently attracting attention as power sources forportable appliances such as portable telephones, PHSs (simplifiedportable telephones), small computers, etc., power sources for powerstorage, and power sources for electric motorcars. In general, a lithiumsecondary battery is constituted of a positive electrode comprising apositive active material as a main component, a negative electrodecomprising a negative-electrode material as a main component, and anonaqueous electrolyte, and is produced by covering a power-generatingelement comprising the positive and negative electrodes with a sheath. Alithium-containing transition metal oxide and a carbonaceous materialare mainly used respectively as the positive active material andnegative-electrode material contained in the lithium secondary battery.Of such materials, a graphite is a material suitable for use inbatteries having a high energy density because it has flat-potentialcharacteristics. Widely known nonaqueous electrolytes are onescomprising an organic solvent comprising ethylene carbonate as a maincomponent and an electrolyte, e.g., lithium hexafluorophosphate (LiPF₆),dissolved in the solvent.

Since the use of ethylene carbonate is apt to cause electrolytesolidification at low temperatures because it has a high melting point,a technique for improving various properties including low-temperatureproperties is known which comprises using an organic solvent having ahigh permittivity and a lower melting point (e.g., propylene carbonate).However, especially in the case of using a graphite in the negativeelectrode, there has been a problem that the organic solvent such aspropylene carbonate decomposes on the graphite negative electrode and,hence, charge/discharge cannot be conducted at a high efficiency.

A technique for overcoming that problem has been disclosed whichcomprises adding vinylene carbonate or the like to a nonaqueouselectrolyte to thereby inhibit the organic-solvent decompositiondescribed above (see, for example, patent document 1). Specifically,there is a statement therein to the effect that the vinylene carbonateor the like is selectively decomposed on the graphite negative electrodein the charge conducted first after battery fabrication (this charge ishereinafter referred to as “first charge”), whereby a protective coatingfilm permeable to lithium ions is formed on the surface of the graphitenegative electrode and the decomposition of the organic solvent such aspropylene carbonate is inhibited. However, this technique wasinsufficient in the effect of inhibiting the decomposition of theorganic solvent during first charge. Furthermore, there has been aproblem that vinylene carbonate has poor oxidation resistance anddecomposes on the positive-electrode side and, hence, the addition ofvinylene carbonate in a large amount reduces battery performances.

On the other hand, quaternary ammonium salts have been frequently usedfor a long time as an electrolyte material for electric double-layercapacitors. However, with respect to application to batteries, the onlytechnique which has been reported is to use a quaternary ammonium saltin a nonaqueous-electrolyte battery which employs a conductive polymer(polyacene) as an electrode material and in which lithium ions do notparticipate in the electrode reactions (see patent documents 2 and 3).No advantage has been found in the use thereof in electrolytes fornonaqueous-electrolyte batteries in which lithium ions participate inelectrode reactions. On the other hand, some of imidazolium salts andthe like which are a kind of quaternary ammonium salt have a property ofroom temperature molten salts, i.e., being liquid at room temperature.It has hence been proposed to use such a salt as a main component of anelectrolyte which eliminates the necessity of using any organic solventsuch as those for use in general nonaqueous-electrolyte batteries. (Seepatent document 4).

[Patent Document 1] JP-A-11-67266

[Patent Document 2] JP-A-62-31958

[Patent Document 3] JP-A-2-177271

[Patent Document 4] JP-A-2002-110230

The invention has been achieved in view of the problems described above.An object of the invention is to provide a nonaqueous-electrolytebattery having a high charge/discharge efficiency and excellenthigh-rate discharge characteristics.

DISCLOSURE OF THE INVENTION

As a result of intensive investigations, the present inventors havesurprisingly found that when a specific nonaqueous electrolyte is used,a nonaqueous-electrolyte battery having a high charge/dischargeefficiency and excellent high-rate discharge characteristics isobtained. The invention has been thus achieved. Namely, the technicalconstitutions of the invention and the effects and advantages thereofare as follows. It is, however, noted that the explanations on themechanisms of the effects include presumptions and whether theseexplanations are correct or not does not limit the scope of theinvention. The invention may be embodied in other various forms withoutdeparting from the spirit or essential characteristics thereof. Theembodiments or Examples described above are therefore to be consideredin all respects as illustrative and not restrictive. The scope of theinvention is indicated by the claims rather than by the description, andall changes and modifications which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

The present inventors made an approach contrary to that used for therelated-art techniques described above in which a quaternary ammoniumsalt is used as a main component. Namely, a system prepared by adding asmall amount of a quaternary ammonium salt to a nonaqueous electrolytecomprising an organic solvent and a lithium salt dissolved therein wasused, and the influence of the quaternary ammonium salt on the behaviorof nonaqueous-electrolyte batteries was closely investigated. As aresult, it has been utterly surprisingly found that the quaternaryammonium salt produces a marked effect on the charge/discharge behaviorof a nonaqueous-electrolyte battery when the concentration of the saltis in a specific range.

Specifically, it has been found according to the invention that theefficiency of the charge/discharge conducted first after batteryfabrication (hereinafter referred to as “initial efficiency”) isremarkably improved even in the case where a graphite is used in thenegative electrode of the nonaqueous-electrolyte battery or even whenthe organic solvent of the nonaqueous electrolyte contains propylenecarbonate or the like. Furthermore, the nonaqueous-electrolyte batteryaccording to the invention has proved to be remarkably improved also inhigh-rate discharge characteristics as compared with the batteriesaccording to related-art techniques.

Namely, the nonaqueous electrolyte of the invention contains a lithiumsalt dissolved therein and is characterized by containing at least onequaternary ammonium salt in an amount of 0.06 mol/L or larger and 0.5mol/L or smaller. The nonaqueous-electrolyte battery of the invention isobtained by using the nonaqueous electrolyte to fabricate a battery. Themechanism by which such constitutions bring about those marked effectson the nonaqueous-electrolyte battery of the invention has not beenfully elucidated. However, it is thought that a protective coating filmpermeable to lithium ions is formed on the surface of the negativeelectrode mainly during first charge due to the use of the nonaqueouselectrolyte of the invention and, because of this, the organic solventconstituting the nonaqueous electrolyte is surely inhibited fromdecomposing. Consequently, it is thought that charge/discharge can beconducted at a high efficiency.

Furthermore, the reduction potentials of many quaternary ammonium saltsare nobler than the reduction potential of vinylene carbonate. It istherefore thought that in the nonaqueous-electrolyte battery accordingto the invention, a protective coating film is surely formed in anearlier stage in first charge than in the nonaqueous-electrolyte batteryemploying vinylene carbonate according to a related-art technique. Inaddition, the protective coating film formed on the surface of thenegative electrode by the decomposition of the quaternary ammonium saltis dense and has excellent permeability to lithium ions. Thus, by merelyadding an only slight amount of a quaternary ammonium salt, the organicsolvent constituting the nonaqueous electrolyte can be more effectivelyinhibited from decomposing. Consequently, a nonaqueous-electrolytebattery having a high charge/discharge efficiency and a high energydensity can be obtained. Incidentally, since quaternary ammonium saltsthemselves have substantially no volatility and have high thermalstability because they are salts, the addition thereof does not impairbattery safety at all.

Namely, the nonaqueous electrolyte of the invention comprises an organicsolvent and a lithium salt dissolved therein, and is characterized bycontaining at least one quaternary ammonium salt in an amount of 0.06mol/L or larger and 0.5 mol/L or smaller. Due to this constitution, anonaqueous electrolyte capable of realizing a battery having a highcharge/discharge efficiency and excellent high-rate dischargecharacteristics can be provided.

The nonaqueous electrolyte of the invention may be characterized in thatthe quaternary ammonium salt has a structure represented by any of(chemical formula 1), (chemical formula 2), and (chemical formula 3):

(wherein R1, R2, R3, and R4 each are either an alkyl group having 1-6carbon atoms or an alkyl group in which at least part of the hydrogenatoms have been replaced by a fluorine atom; and X⁻ is afluorine-containing anion)

(wherein R is a divalent organic linking group having a main chain whichhas 4-5 atoms and is constituted of at least one member selected fromcarbon, oxygen, nitrogen, sulfur, and phosphorus; R1 and R2 each areeither an alkyl group having 1-6 carbon atoms or an alkyl group in whichat least part of the hydrogen atoms have been replaced by a fluorineatom; and X⁻ is a fluorine-containing anion)

(wherein R is an organic linking group or an organic linking groupforming an aromatic ring, the organic linking groups each having a mainchain which has 4-5 atoms and is constituted of at least one memberselected from carbon, oxygen, nitrogen, sulfur, and phosphorus andhaving one single-bond end and one double-bond end; R1 is an alkyl grouphaving 1-6 carbon atoms or an alkyl group in which at least part of thehydrogen atoms have been replaced by a fluorine atom; and X⁻ is afluorine-containing anion).

Due to this constitution, a nonaqueous electrolyte can be provided whichenables a lithium ion-permeable protective coating film which is denserand has higher permeability to lithium ions to be formed during firstcharge on the surface of the negative electrode of the battery employingthe electrolyte to thereby effectively inhibit the decomposition of theorganic solvent constituting the nonaqueous electrolyte, and whichenables the battery to be sufficiently charged/discharged in the secondand subsequent cycles and thereby have an improved charge/dischargeefficiency.

Furthermore, the nonaqueous electrolyte of the invention may becharacterized by containing one or more organic solvents selected fromthe group consisting of ethylene carbonate, propylene carbonate,butylene carbonate, γ-butyrolactone, and γ-valerolactone.

Due to this constitution, the battery employing this nonaqueouselectrolyte can have a remarkably improved initial efficiency althoughsuch organic solvent is used. Consequently, a nonaqueous electrolyte fornonaqueous-electrolyte batteries can be provided which sufficientlytakes advantage of properties of those solvents, i.e., the properties ofhaving a high boiling point and a high flash point and having a highpermittivity and excellent oxidation resistance.

Moreover, the nonaqueous electrolyte of the invention may becharacterized in that the anion species contained in the nonaqueouselectrolyte is one or more members selected from the group consisting ofBF₄ ⁻, PF₆ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, N(C₂F₅SO₂)₂ ⁻,N(CF₃SO₂)(C₄F₉SO₂)⁻; C(CF₃SO₂)₃ ⁻, and C(C₂F₅SO₂)₃ ⁻.

In this constitution, the anion species of the lithium salt orquaternary ammonium salt is one containing fluorine atoms. The salthence readily dissolves in the nonaqueous electrolyte. Consequently, theeffects described above are effectively obtained. In particular, byselecting the anion species from the group shown above, anonaqueous-electrolyte battery combining high performances and excellentsafety can be obtained because those fluorine-containing anions arestable over a wide potential range. Those anion species may be containedalone, or two or more thereof may be simultaneously contained.

The nonaqueous-electrolyte battery of the invention comprises a positiveelectrode, a negative electrode, and a nonaqueous electrolyte, and ischaracterized by having been fabricated using the nonaqueous electrolytedescribed above. Due to this constitution, a nonaqueous-electrolytebattery can be provided in which the effects of the invention describedabove are produced.

Furthermore, the nonaqueous-electrolyte battery of the invention may becharacterized in that the negative electrode employs a graphite. Due tothis constitution, although a graphite is used as a negative-electrodematerial, first charge can be conducted while effectively inhibiting thedecomposition of the organic solvent constituting the nonaqueouselectrolyte. This constitution further brings about an improvement inhigh-rate discharge characteristics. Consequently, by using a graphitein the negative electrode, a nonaqueous-electrolyte battery can beprovided which sufficiently takes advantage of that property of agraphite negative electrode material which is the property of showing aflat potential change to enable a high energy density.

Moreover, the nonaqueous-electrolyte battery of the invention may becharacterized by having a sheath comprising a metal/resin compositematerial. In this constitution, even though the sheath is made of aflexible material, there is no possibility that the battery might swellduring charge because the nonaqueous electrolyte in the battery systemof the invention can be effectively inhibited from decomposing duringcharge due to the functions described above and, hence, almost no gasgeneration occurs during charge. Consequently, a sheath comprising alightweight metal/resin composite material can be employed and anonaqueous-electrolyte battery having a further improved energy densitycan hence be provided.

It is thought that in the nonaqueous-electrolyte battery according tothe invention, part of the quaternary ammonium salt is consumed by thereaction for forming a protective coating film in a first charge step.There are hence cases where the concentration of the quaternary ammoniumsalt in the nonaqueous electrolyte present in the nonaqueous-electrolytebattery according to the invention after the first charge step is lowerthan the concentration of the quaternary ammonium salt in the nonaqueouselectrolyte of the invention used in the battery.

Embodiments of the invention will be shown below, but the inventionshould not be construed as being limited by the following statements.

The quaternary ammonium salt to be used in the invention more preferablyhas a structure represented by any of (chemical formula 1), (chemicalformula 2), and (chemical formula 3). Examples of the quaternaryammonium salt represented by (chemical formula 1) include quaternaryammonium salts such as (CH₃)₄NBF₄, (CH₃)₄NBr, (CH₃)₄N(CF₃SO₂)₂N,(CH₃)₄N(C₂F₅SO₂)₂N, (C₂H₅)₄NBF₄, (C₂H₅)₄NClO₄, (C₂H₅)₄NI,(C₂H₅)₄N(CF₃SO₂)₂N, (C₂H₅)₄N(C₂F₅SO₂)₂N, (C₃H₇)₄NBr, (n-C₄H₉)₄NBF₄,(n-C₄H₉)₄N(CF₃SO₂)₂N, (n-C₄H₉)₄N(C₂F₅SO₂)₂N, (n-C₄H₉)₄NClO₄,(n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, and (C₂H₅)₄N-phtalate.However, the quaternary ammonium salt represented by (chemicalformula 1) should not be construed as being limited to these.

Examples of the quaternary ammonium salt represented by (chemicalformula 2) include quaternary ammonium salts comprising a combination ofa pyrrolidinium cation, piperidinium cation, pyrrolium cation, or thelike and an anion. However, quaternary ammonium salt represented by(chemical formula 2) should not be construed as being limited to these.

Examples of the pyrrolidinium cation include a 1,1-dimethylpyrrolidiniumion, 1-ethyl-1-methylpyrrolidinium ion, 1-methyl-1-propylpyrrolidiniumion, and 1-butyl-1-methylpyrrolidinium ion. However, the pyrrolidiniumcation should not be construed as being limited to these.

Examples of the piperidinium cation include a 1,1-dimethylpiperidiniumion, 1-ethyl-1-methylpiperidinium ion, 1-methyl-1-propylpiperidiniumion, and 1-butyl-1-methylpiperidinium ion. However, the piperidiniumcation should not be construed as being limited to these. Examples ofthe pyrrolium cation include a 1,1-dimethylpyrrolium ion,1-ethyl-1-methylpyrrolium ion, 1-methyl-1-propylpyrrolium ion, and1-butyl-1-methylpyrrolium ion. However, the pyrrolium cation should notbe construed as being limited to these.

Examples of the quaternary ammonium salt represented by (chemicalformula 3) include quaternary ammonium salts comprising a combination ofan imidazolium cation, pyrazolium cation, pyrrolinium cation, pyridiniumcation, or the like and an anion. However, the quaternary ammonium saltrepresented by (chemical formula 3) should not be construed as beinglimited to these.

Examples of the imidazolium cation include a 1,3-dimethylimidazoliumion, 1-ethyl-3-methylimidazolium ion, 1-butyl-3-methylimidazolium ion,1,2,3-trimethylimidazolium ion, 1,2-dimethyl-3-ethylimidazolium ion,1,2-dimethyl-3-propylimidazolium ion, and1-butyl-2,3-dimethylimidazolium ion. However, the imidazolium cationshould not be construed as being limited to these. Examples of thepyrazolium cation include a 1,2-dimethylpyrazolium ion,1-ethyl-2-methylpyrazolium ion, 1-propyl-2-methylpyrazolium ion, and1-butyl-2-methylpyrazolium ion. However, the pyrazolium cation shouldnot be construed as being limited to these. Examples of the pyrroliniumcation include a 1,2-dimethylpyrrolinium ion,1-ethyl-2-methylpyrrolinium ion, 1-propyl-2-methylpyrrolinium ion, and1-butyl-2-methylpyrrolinium ion. However, the pyrrolinium cation shouldnot be construed as being limited to these. Examples of the pyridiniumcation include an N-methylpyridinium ion, N-ethylpyridinium ion,N-propylpyridinium ion, N-butylpyridinium ion,1-ethyl-2-methylpyridinium, 1-butyl-4-methylpyridinium, and1-butyl-2,4-dimethylpyridinium. However, the pyridinium cation shouldnot be construed as being limited to these.

Examples of the anions include a chlorine anion, bromine anion, ClO₄anion, BF₄ anion, PF₆ anion, CF₃SO₃ anion, N(CF₃SO₂)₂ anion, N(C₂F₅SO₂)₂anion, N(CF₃SO₂)(C₄F₉SO₂) anion, C(CF₃SO₂)₃ anion, and C(C₂F₅SO₂)₃anion. However, the anions should not be construed as being limited tothese.

Those quaternary ammonium salts can be used alone or as a mixture of twoor more thereof.

The amount of the quaternary ammonium salt to be incorporated in thenonaqueous electrolyte of the invention is 0.06 mol/L or larger and 0.5mol/L or smaller based on the whole amount of the nonaqueouselectrolyte. Preferably, the amount thereof is 0.1-0.35 mol/L. In casewhere the amount of the quaternary ammonium salt contained is smallerthan 0.06 mol/L based on the whole amount of the nonaqueous electrolyte,the organic solvent constituting the nonaqueous electrolyte cannot besufficiently inhibited from decomposing during first charge, making itdifficult to sufficiently charge the battery. On the other hand, in casewhere the amount of the quaternary ammonium salt contained exceeds 0.5mol/L, it is impossible to sufficiently heighten the charge efficiencyand high-rate discharge characteristics. The reasons for this may bebecause the nonaqueous electrolyte comes to have an increased viscosityand the protective coating film comes to have increased resistance,although presumptions are included.

As the organic solvent constituting the nonaqueous electrolyte can beused an organic solvent generally used in nonaqueous electrolytes fornonaqueous-electrolyte batteries. Examples thereof include cycliccarbonates such as propylene carbonate, ethylene carbonate, butylenecarbonate, chloroethylene carbonate, vinylene carbonate, andvinylethylene carbonate; cyclic esters such as γ-butyrolactone,γ-valerolactone, and propiolactone; chain carbonates such as dimethylcarbonate, diethyl carbonate, ethyl methyl carbonate, and diphenylcarbonate; chain esters such as methyl acetate and methyl lactate;ethers such as tetrahydrofuran or derivatives thereof, 1,3-dioxane,dimethoxyethane, diethoxyethane, methoxyethoxyethane, and methyldiglyme;nitrites such as acetonitrile and benzonitrile; dioxolane or derivativesthereof; and sulfolane, sultones, or derivatives thereof. These may beused alone or as a mixture or the like of two or more thereof. Theorganic solvent should not be construed as being limited to theseexamples.

It is especially preferred in the invention that the nonaqueouselectrolyte should contain at least one member selected from ethylenecarbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, andγ-valerolactone, because this nonaqueous electrolyte can sufficientlyproduce the effects of the invention.

Examples of the electrolyte salt include inorganic ionic saltscontaining one of lithium (Li), sodium (Na), and potassium (K), such asLiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀,NaClO₄, NaI, NaSCN, NaBr, KClO₄, and KSCN; and organic ionic salts suchas LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂),LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, lithium stearylsulfonate, lithiumoctylsulfonate, and lithium dodecylbenzenesulfonate. These ioniccompounds can be used alone or as a mixture of two or more thereof.

It is especially preferred in the invention that one or more lithiumsalts selected from LiBF₄, LiPF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, and LiC(C₂F₅SO₂)₃ be contained asthe electrolyte salt(s). In particular, use of a mixture of one or moresalts selected from the group consisting of LiPF₆ and LiBF₄ with one ormore salts selected from the group consisting of lithium salts havingone or more perfluoroalkyl groups, such as LiN(CF₃SO₂)₂ andLiN(C₂F₅SO₂)₂, is preferred because this mixture has the effect ofimproving the storability of the nonaqueous-electrolyte battery.

The concentration of the electrolyte salt in the nonaqueous electrolyteis preferably from 0.1 mol/L to 5 mol/L, more preferably from 1 mol/L to2.5 mol/L, from the standpoint of surely obtaining anonaqueous-electrolyte battery having high battery characteristics. Themixing ratio between the electrolyte salt and the quaternary ammoniumsalt is such that the electrolyte salt/quaternary ammonium salt ratio bymole is preferably 50/50 or higher, more preferably 70/30 or higher.

The positive active material to be used as a main component of thepositive electrode desirably is any one of or a mixture of two or moreof lithium-containing transition metal oxides, lithium-containingphosphoric acid salts, lithium-containing sulfuric acid salts, and thelike. Examples of the lithium-containing transition metal oxides includeLi—Co composite oxides and Li—Mn composite oxides. Such oxides in whichpart of the cobalt or manganese has been replaced by one or more metalsin Groups I to VIII of the periodic table (preferably one or moreelements selected from the group consisting of, e.g., Li, Ca, Cr, Ni,Mn, Fe, and Co) can also be advantageously used. Examples of the Li—Mncomposite oxides include one having a spinel crystal structure and onehaving an α-NaFeO₂ type crystal structure, and either of these can beadvantageously used. According to battery designs, a suitable one can beselected from these lithium-containing transition metal oxides or amixture thereof may be used.

A mixture of any of those lithium-containing compounds with anotherpositive active material may be used. Examples of the positive activematerial usable besides the lithium-containing compounds includecompounds of a Group I metal, such as CuO, Cu₂O, Ag₂O, CuS, and CuSO₄,compounds of a Group IV metal, such as TiS₂, SiO₂, and SnO, compounds ofa Group V metal, such as V₂O₅, V₆O₁₂, VO_(x), Nb₂O₅, Bi₂O₃, and Sb₂O₃,compounds of a Group VI metal, such as CrO₃, Cr₂O₃, MoO₃, MoS₂, WO₃, andSeO₂, compounds of a Group VII metal, such as MnO₂ and Mn₂O₃, compoundsof a Group VIII metal, such as Fe₂O₃, FeO, Fe₃O₄, Ni₂O₃, NiO, CoO₃, andCoO, and metal compounds represented by the general formula Li_(x)MX₂ orLi_(x)MN_(y)X₂ (wherein M and N each represent a metal in Groups I toVIII, and X represents a chalcogen atom such as oxygen or sulfur) suchas, e.g., lithium-cobalt composite oxides and lithium-manganesecomposite oxides. Examples thereof further include conductive polymericcompounds, such as disulfides, polypyrrole, polyaniline,poly-p-phenylene, polyacetylene, and polyacene materials, andcarbonaceous materials of the pseudo-graphite structure. However, usablepositive active materials should not be construed as being limited tothese examples.

Examples of the negative-electrode material to be used as a maincomponent of the negative electrode include carbonaceous materials,metal oxides such as tin oxides and silicon oxides, and materialsobtained by modifying these materials by adding phosphorus or boron forthe purpose of improving negative-electrode properties. Of carbonaceousmaterials, graphites have an operating potential very close to that ofmetallic lithium and are hence effective in diminishing self-dischargewhen a lithium salt is employed as an electrolyte. Furthermore,graphites are effective in reducing the irreversible capacity incharge/discharge. Consequently, graphites are preferrednegative-electrode materials. In addition, since the nonaqueouselectrolyte containing a quaternary ammonium salt is used in theinvention, the organic solvent constituting the nonaqueous electrolytecan be surely inhibited from decomposing on the negative electrodecomprising a graphite as a main component during charge. Theadvantageous properties of the graphite shown above can hence be surelyexhibited.

Results of examinations by X-ray diffractometry, etc. of graphites whichcan be advantageously used are shown below.

Lattice spacing (d₀₀₂): 0.333-0.350 nm

Crystallite size in a-axis direction, La: ≧90 nm

Crystallite size in c-axis direction, Lc: ≧20 nm

True density: 2.00-2.25 g/cm³

It is also possible to modify a graphite by adding thereto a metaloxide, e.g., tin oxide or silicon oxide, phosphorus, boron, amorphouscarbon, or the like. In particular, modifying the surface of a graphiteby the method described above is desirable because this modification caninhibit electrolyte decomposition and thereby heighten batterycharacteristics. Furthermore, a combination of a graphite and eitherlithium metal or a lithium metal-containing alloy, such aslithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin,lithium-gallium, or Wood's metal, or the like can be used as anegative-electrode material. A graphite into which lithium has beeninserted beforehand by electrochemical reduction can also be used as anegative-electrode material.

Furthermore, at least a surface layer part of a powder of the positiveactive material and a powder of the negative-electrode material may bemodified with a substance having satisfactory electron conductivity orion conductivity or with a compound having a hydrophobic group. Examplesof this modification include the deposition of a substance havingsatisfactory electron conductivity, such as gold, silver, carbon,nickel, or copper, a substance having satisfactory ion conductivity,such as lithium carbonate, a boron-containing glass, or a solidelectrolyte, or a substance having a hydrophobic group, such as asilicone oil, by a technique such as plating, sintering, mechanofusion,vapor deposition, baking, etc.

A powder of the positive active material and a powder of thenegative-electrode material desirably have an average particle size of100 μm or smaller. In particular, it is desirable that the averageparticle size of the powder of the positive active material be 10 m orsmaller for the purpose of improving the high-output characteristics ofthe nonaqueous-electrolyte battery. A pulverizer and a classifier areused for obtaining a powder having a given size. For example, use ismade of a mortar, ball mill, sand mill, oscillating ball mill, planetaryball mill, jet mill, counter jet mill, or cyclone type jet mill andsieves or the like. Pulverization may be conducted by wet pulverizationin which water or an organic solvent, e.g., hexane, coexists. Methods ofclassification are not particularly limited, and sieves, an airclassifier, or the like is used in each of dry and wet processesaccording to need.

Although the positive active material and the negative-electrodematerial were described above in detail, the positive electrode andnegative electrode may contain a conductive material, a binder, and afiller as other components besides the active materials as maincomponents.

The conductive material is not limited as long as it is anelectron-conductive material not adversely influencing batteryperformances. Usually, however, conductive materials such as naturalgraphite (e.g., flake graphite, flaky graphite, or soil-like graphite),artificial graphite, carbon black, acetylene black, Ketjen Black, carbonwhiskers, carbon fibers, metal (e.g., copper, nickel, aluminum, silver,or gold) powders, metal fibers, and conductive ceramic materials can beincorporated alone or as a mixture thereof.

A preferred conductive material of these is acetylene black from thestandpoints of conductivity and applicability. The amount of theconductive material to be added is preferably from 1% by weight to 50%by weight, especially preferably from 2% by weight to 30% by weight,based on the total weight of the positive electrode or negativeelectrode. For mixing those ingredients, physical mixing is conducted.Homogeneous mixing is ideal. For this mixing, a powder mixer such as aV-type mixer, S-type mixer, mortar mill, ball mill, or planetary millcan be used in a dry or wet mixing process.

As the binder can usually be used one of or a mixture of two or more ofthermoplastic resins such as polytetrafluoroethylene, poly(vinylidenefluoride), polyethylene, and polypropylene, polymers having rubberelasticity, such as ethylene/propylene/diene terpolymers (EPDM),sulfonated EPDM, styrene/butadiene rubbers (SBR), and fluororubbers,polysaccharides such as carboxymethyl cellulose, and the like. In thecase of a binder having functional groups reactive with lithium, such aspolysaccharides, it is desirable to deactivate the functional groupsbeforehand by, e.g., methylation. The amount of the binder to be addedis preferably 1-50% by weight, especially preferably 2-30% by weight,based on the total weight of the positive electrode or negativeelectrode.

As the filler may be used any material which does not adverselyinfluence battery performances. Usually, use is made of an olefinpolymer such as polypropylene or polyethylene, a metal oxide such assilicon oxide, titanium oxide, aluminum oxide, magnesium oxide,zirconium oxide, zinc oxide, or iron oxide, a metal carbonate such ascalcium carbonate or magnesium carbonate, a glass, carbon, etc. Theamount of the filler to be added is preferably up to 30% by weight basedon the total weight of the positive electrode or negative electrode.

The positive electrode and negative electrode are produced preferably bymixing the active material, a conductive material, and a binder with anorganic solvent, e.g., N-methylpyrrolidone or toluene, subsequentlyapplying the resultant liquid mixture to the current collector whichwill be described below, and then drying the coating. In theapplication, it is desirable to apply the liquid mixture, for example,by roller coating using an applicator roll, screen coating, doctor bladecoating, spin coating, or coating with a bar coater or the like in anydesired thickness and any desired shape. However, methods of applicationshould not be construed as being limited to these.

As the current collector may be used any electron conductor which doesnot exert an adverse influence in the battery fabricated. For example,the current collector for the positive electrode can be aluminum,titanium, stainless steel, nickel, burned carbon, a conductive polymer,conductive glass, or the like. Besides these, use can be made, as thepositive-electrode current collector, of a material obtained by treatingthe surface of aluminum, copper, or the like with carbon, nickel,titanium, silver, or the like for the purpose of improving adhesiveness,conductivity, and reduction resistance. The current collector for thenegative electrode can be copper, nickel, iron, stainless steel,titanium, aluminum, burned carbon, a conductive polymer, conductiveglass, Al—Cd alloy, or the like. Besides these, use can be made, as thenegative-electrode current collector, of a material obtained by treatingthe surface of copper or the like with carbon, nickel, titanium, silver,or the like for the purpose of improving adhesiveness, conductivity, andoxidation resistance. These materials can be subjected to a surfaceoxidation treatment.

With respect to the shape of the current collector, use is made of afoil form or a film, sheet, net, punched or expanded, lath, porous, orfoamed form. A structure made up of fibers is also usable. Although thethickness thereof is not particularly limited, collectors having athickness of 1-500 μm are used. Of these current collectors, a preferredcollector for the positive electrode is an aluminum foil, which hasexcellent oxidation resistance. Preferred current collectors for thenegative electrode are a copper foil, nickel foil, and iron foil, whichare stable in a reducing field, have excellent electrical conductivity,and are inexpensive, and an alloy foil containing part of these.Furthermore, these foils preferably are ones in which the rough-surfaceside has a surface roughness Ra of 0.2 μm or more. This surfaceroughness enables the current collector to retain excellent adhesion tothe positive active material or negative-electrode material. It istherefore preferred to use an electrolytic foil, which has such a roughsurface. Most preferred is an electrolytic foil which has undergone a“hana” surface treatment.

The separator for nonaqueous-electrolyte batteries preferably is one ofor a combination of two or more of microporous films, nonwoven fabrics,and the like which show excellent rate characteristics. Examples of thematerial constituting the separator for nonaqueous-electrolyte batteriesinclude polyolefin resins represented by polyethylene and polypropylene,polyester resins represented by poly(ethylene terephthalate) andpoly(butylene terephthalate), poly(vinylidene fluoride), vinylidenefluoride/hexafluoropropylene copolymers, vinylidenefluoride/perfluorovinyl ether copolymers, vinylidenefluoride/tetrafluoroethylene copolymers, vinylidenefluoride/trifluoroethylene copolymers, vinylidenefluoride/fluoroethylene copolymers, vinylidenefluoride/hexafluoroacetone copolymers, vinylidene fluoride/ethylenecopolymers, vinylidene fluoride/propylene copolymers, vinylidenefluoride/trifluoropropylene copolymers, vinylidenefluoride/tetrafluoroethylene/-hexafluoropropylene copolymers, andvinylidene fluoride/ethylene/tetrafluoroethylene copolymers.

The porosity of the separator for nonaqueous-electrolyte batteries ispreferably 98% by volume or lower from the standpoint of strength. Theporosity thereof is preferably 20% by volume or higher from thestandpoint of charge/discharge characteristics.

As the separator for nonaqueous-electrolyte batteries may be used apolymer gel constituted of a polymer of, e.g., acrylonitrile, ethyleneoxide, propylene oxide, methyl methacrylate, vinyl acetate,vinylpyrrolidone, vinylidene fluoride, or the like and an electrolyte.

Furthermore, a separator for nonaqueous-electrolyte batteries whichcomprises a combination of a porous film, nonwoven fabric, or the likesuch as that described above and a polymer gel is desirable because useof this separator improves electrolyte retention. Namely, the surface ofa microporous polyethylene film and the walls of the micropores arecoated in a thickness of up to several micrometers with a polymer havingaffinity for solvents and an electrolyte is held in the micropores ofthe coated film, whereby the polymer having affinity for solvents gels.

Examples of the polymer having affinity for solvents includepoly(vinylidene fluoride) and polymers formed by the crosslinking of anacrylate monomer having an ethylene oxide group or ester group, epoxymonomer, monomer having isocyanate groups, or the like. Heat, actinicrays such as ultraviolet (UV) or electron beams (EB), or the like can beused for the crosslinking.

For the purpose of regulating strength or properties, a propertyregulator can be incorporated into the polymer having affinity forsolvents in such an amount as not to inhibit the formation of acrosslinked structure. Examples of the property regulator includeinorganic fillers {metal oxides such as silicon oxide, titanium oxide,aluminum oxide, magnesium oxide, zirconium oxide, zinc oxide, and ironoxide and metal carbonates such as calcium carbonate and magnesiumcarbonate} and polymers {poly(vinylidene fluoride), vinylidenefluoride/hexafluoropropylene copolymers, polyacrylonitrile, poly(methylmethacrylate), and the like}. The amount of the property regulator to beadded is generally up to 50% by weight, preferably up to 20% by weight,based on the crosslinkable monomer.

Examples of the acrylate monomer include unsaturated monomers having afunctionality of 2 or higher. Specific examples thereof includedifunctional (meth)acrylates {ethylene glycol di(meth)acrylate,propylene glycol di(meth)acrylate, adipic acid dineopentyl glycol esterdi(meth)acrylate, polyethylene glycol di(meth)acrylates having a degreeof polymerization of 2 or higher, polypropylene glycol di(meth)acrylateshaving a degree of polymerization of 2 or higher,polyoxyethylene/polyoxypropylene copolymer di(meth)acrylates, butanedioldi(meth)acrylate, hexamethylene glycol di(meth)acrylate, and the like},trifunctional (meth)acrylates {trimethylolpropane tri(meth)acrylate,glycerol tri(meth)acrylate, tri(meth)acrylates of ethylene oxide adductsof glycerol, tri(meth)acrylates of propylene oxide adducts of glycerol,tri(meth)acrylates of ethylene oxide/propylene oxide adducts ofglycerol, and the like}, and (meth)acrylates having a functionality of 4or higher {pentaerythritol tetra(meth)acrylate, diglycerolhexa(meth)acrylate, and the like}. These monomers can be used alone orin combination.

A monofunctional monomer may be added to the acrylate monomer for thepurpose of property regulation, etc. Examples of the monofunctionalmonomer include unsaturated carboxylic acids {acrylic acid, methacrylicacid, crotonic acid, cinnamic acid, vinylbenzoic acid, maleic acid,fumaric acid, itaconic acid, citraconic acid, mesaconic acid,methylmalonic acid, aconitic acid, and the like}; unsaturated sulfonicacids {styrenesulfonic acid, acrylamido-2-methylpropanesulfonic acid,and the like} or salts of these (lithium salts, sodium salts, potassiumsalts, ammonium salts, tetraalkylammonium salts, and the like); thoseunsaturated carboxylic acids partly esterified with a C₁-C₁₈ aliphaticor alicyclic alcohol, alkylene (C₂-C₄) glycol, polyalkylene (C₂-C₄)glycol, or the like (methyl maleate, monohydroxyethyl maleate, and thelike) or partly amidated with ammonia or a primary or secondary amine(maleic acid monoamide, N-methylmaleic acid monoamide, N,N-diethylmaleicacid monoamide, and the like); (meth) acrylic esters [esters of(meth)acrylic acid with a C₁-C₁₈ aliphatic (e.g., methyl, ethyl, propyl,butyl, 2-ethylhexyl, or stearyl) alcohol; and esters of (meth)acrylicacid with an alkylene (C₂-C₄) glycol (ethylene glycol, propylene glycol,1,4-butanediol, or the like) or with a polyalkylene (C₂-C₄) glycol(polyethylene glycol or polypropylene glycol)]; (meth)acrylamide orN-substituted (meth)acrylamides [(meth)acrylamide,N-methyl(meth)acrylamide, N-methylol(meth)acrylamide, and the like];vinyl esters or allyl esters [vinyl acetate, allyl acetate, and thelike]; vinyl ethers or allyl ethers [butyl vinyl ether, dodecyl allylether, and the like]; unsaturated nitrile compounds[(meth)acrylonitrile, crotononitrile, and the like]; unsaturatedalcohols [(meth)allyl alcohol and the like]; unsaturated amines[(meth)allylamine, dimethylaminoethyl (meth)acrylate,diethylaminoethyl(meth)acrylate, and the like]; heterocycle-containingmonomers [N-vinylpyrrolidone, vinylpyridine, and the like]; olefinicaliphatic hydrocarbons [ethylene, propylene, butylene, isobutylene,pentene, (C₆-C₅₀) α-olefins, and the like]; olefinic alicyclichydrocarbons [cyclopentene, cyclohexene, cycloheptene, norbornene, andthe like]; olefinic aromatic hydrocarbons [styrene, α-methylstyrene,stilbene, and the like]; unsaturated imides [maleimide and the like];and halogen-containing monomers [vinyl chloride, vinylidene chloride,vinylidene fluoride, hexafluoropropylene, and the like].

Examples of the epoxy monomer include glycidyl ethers {bisphenol Adiglycidyl ether, bisphenol F diglycidyl ether, brominated bisphenol Adiglycidyl ether, phenol-novolac glycidyl ether, cresol-novolac glycidylether, and the like}, glycidyl esters {hexahydrophthalic acid glycidylester, dimer acid glycidyl esters, and the like}, glycidylamines{triglycidyl isocyanurate, tetraglycidyldiaminophenylmethane, and thelike}, linear aliphatic epoxides {epoxidized polybutadiene, epoxidizedsoybean oil, and the like}, and alicyclic epoxides{3,4-epoxy-6-methylcyclohexylmethylcarboxylate,3,4-epoxycyclohexylmethylcarboxylate, and the like}. These epoxy resinscan be used alone or after having been cured by addition of a hardenerthereto. Examples of the hardener include aliphatic polyamines{diethylenetriamine, triethylenetetramine,3,9-(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5,5]undecane, and the like},aromatic polyamines {m-xylenediamine, diaminophenylmethane, and thelike}, polyamides {dimer acid polyamides and the like}, acid anhydrides{phthalic anhydride, tetrahydromethylphthalic anhydride,hexahydrophthalic anhydride, trimellitic anhydride, and methylnadicanhydride}, phenol compounds {phenolic novolacs and the like},polymercaptans {polysulfides and the like}, tertiary amines{tris(dimethylaminomethyl)phenol, 2-ethyl-4-methylimidazole, and thelike}, and Lewis acid complexes {boron trifluoride/ethylamine complexand the like}.

Examples of the monomer having isocyanate groups include toluenediisocyanate, diphenylmethane diisocyanate, 1,6-hexamethylenediisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, p-phenylenediisocyanate, 4,4′-dicyclohexylmethane diisocyanate,3,3′-dimethyldiphenyl 4,4′-diisocyanate, dianisidine diisocyanate,m-xylene diisocyanate, trimethylxylene diisocyanate, isophoronediisocyanate, 1,5-naphthalene diisocyanate, trans-1,4-cyclohexyldiisocyanate, and lysine diisocyanate.

In crosslinking the monomer having isocyanate groups, a compound havingactive hydrogen may also be used. Examples of this compound includepolyols and polyamines [difunctional compounds {water, ethylene glycol,propylene glycol, diethylene glycol, dipropylene glycol, and the like},trifunctional compounds {glycerol, trimethylolpropane,1,2,6-hexanetriol, triethanolamine, and the like}, tetrafunctionalcompounds {pentaerythritol, ethylenediamine, tolylenediamine,diphenylmethanediamine, tetramethylolcyclohexane, methylglucosides, andthe like}, pentafunctional compounds{2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol, diethylenetriamine, andthe like}, hexafunctional compounds {sorbitol, mannitol, dulcitol, andthe like}, and octafunctional compounds {sucrose and the like}],polyether polyols {propylene oxide and/or ethylene oxide adducts of thepolyols or polyamines mentioned above}, and polyester polyols[condensates of the aforementioned polyols with a polybasic acid {adipicacid, o-, m-, or p-phthalic acid, succinic acid, azelaic acid, sebacicacid, or ricinoleic acid}, polycaprolactone polyols {poly-ε-caprolactoneand the like}, polycondensates of hydroxycarboxylic acids, and thelike].

A catalyst may also be used in conducting the crosslinking reaction.Examples of the catalyst include organotin compounds,trialkylphosphines, amines [monoamines {N,N-dimethylcyclohexylamine,triethylamine, and the like}, cyclic monoamines {pyridine,N-methylmorpholine, and the like}, diamines{N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-butanediamine, and the like}, triamines{N,N,N′,N′-pentamethyldiethylenetriamine and the like}, hexamines{N,N,N′,N′-tetra(3-dimethylaminopropyl)methanediamine and the like},cyclic polyamines {diazabicyclooctane (DABCO), N,N′-dimethylpiperazine,1,2-dimethylimidazole, 1,8-diazabicyclo(5,4,0)undecene-7 (DBU), and thelike}, and salts of these.

In the step of battery fabrication, methods or procedures for applyingthe nonaqueous electrolyte of the invention are not limited. Use may bemade of a method in which a power-generating element comprising apositive electrode, a negative electrode, and a separator is assembledfirst and then immersed in and impregnated with the liquid nonaqueouselectrolyte, which is caused to gel after the impregnation in somecases. Alternatively, a power-generating element may be assembled afterthe nonaqueous electrolyte is infiltrated into a positive electrode ornegative electrode (and caused to gel after the infiltration in somecases). With respect to methods for immersion, the power-generatingelement may be immersed at ordinary pressure. However, the vacuumimmersion method or the pressure immersion method can also be used.Furthermore, a positive electrode or negative electrode may be formed bykneading materials for the nonaqueous electrolyte together with anelectrode material and applying the resultant mixture. This method canbe used, for example, in the case where the nonaqueous electrolyte is,in particular, a polymeric solid electrolyte.

The sheath preferably is constituted of a thin material from thestandpoint of reducing the weight of the nonaqueous-electrolyte battery.For example, a metal/resin composite material having a constitutioncomprising resin films and a metal foil sandwiched therebetween ispreferred. Examples of the metal foil are not particularly limited aslong as they are foils of aluminum, iron, nickel, copper, stainlesssteel, titanium, gold, silver, or the like which are free from pinholes.However, aluminum foils are preferred because they are lightweight andinexpensive. Preferred for use as the resin film to be disposed on theouter side in the battery is a resin film having excellent piercingstrength, such as a poly(ethylene terephthalate) film or nylon film.Preferred as the resin film to be disposed on the inner side in thebattery is a film which is fusion-bondable and has solvent resistance,such as a polyethylene film or nylon film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a nonaqueous-electrolyte battery accordingto Examples of the invention.

FIG. 2 is a presentation showing performances of batteries of theinvention and comparative batteries.

Numeral 1 denotes a positive electrode, 11 a positive composite, 12 apositive-electrode current collector, 2 a negative electrode, 21 anegative composite, 22 a negative-electrode current collector, 3 aseparator, 4 a power-generating element, and 5 a metal/resin compositefilm.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained below in more detail by reference toExamples, but the invention should not be construed as being limited tothese statements.

Example 1

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Furthermore, tetraethylammoniumtetrafluoroborate ((C₂H₅)₄NBF₄) was mixed therewith in an amount of 0.06mol/L. Thus, a nonaqueous electrolyte was obtained.

Example 2

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Furthermore, trimethyl-n-butylammoniumubis(trifluoromethylsulfonyl)imide((CH₃)₃(n-C₄H₉)N(CF₃SO₂)₂N) was mixedtherewith in an amount of 0.1 mol/L. Thus, a nonaqueous electrolyte wasobtained.

Example 3

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Furthermore, 1-ethyl-3-methylimidazoliumbis(perfluoroethylsulfonyl)imide was mixed therewith in an amount of 0.3mol/L. Thus, a nonaqueous electrolyte was obtained.

Example 4

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Furthermore, 1-butylpyridiniumhexafluorophosphate was mixed therewith in an amount of 0.5 mol/L. Thus,a nonaqueous electrolyte was obtained.

COMPARATIVE EXAMPLE 1

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Furthermore, (C₂H₅)₄NBF₄ was mixed therewithin an amount of 0.7 mol/L. Thus, a nonaqueous electrolyte was obtained.

COMPARATIVE EXAMPLE 2

One mole of LiPF₆ was dissolved in 1 L of a mixed solvent prepared bymixing ethylene carbonate, propylene carbonate, and diethyl carbonate ina ratio of 6:2:2 by volume. Thus, a nonaqueous electrolyte was obtained.

(Batteries of the Invention and Comparative Batteries)

The nonaqueous electrolytes of Examples 1 to 4 and Comparative Examples1 and 2 given above were used to produce lithium batteries asnonaqueous-electrolyte batteries.

The lithium batteries according to the Examples are constituted of: apower-generating element 4 comprising a positive electrode 1, a negativeelectrode 2, and a separator 3; a nonaqueous electrolyte; and ametal/resin composite film 5 as a sheath. The positive electrode 1comprises a positive-electrode current collector 12 and a positivecomposite 11 applied thereon. The negative electrode 2 comprises anegative-electrode current collector 22 and a negative composite 21applied thereon. The nonaqueous electrolyte has been infiltrated intothe power-generating element 4. The metal/resin composite film 5 hasbeen disposed so as to cover the power-generating element 4, with thefour sides thereof sealed by thermal fusion bonding. A sectional view ofthe lithium batteries according to the Examples is shown in FIG. 1. Theprocess used for producing the batteries of the constitution describedabove is explained below.

The positive electrode 1 was obtained in the following manner. First,LiCoO₂ as a positive active material was mixed with acetylene black as aconductive material. Furthermore, an N-methyl-2-pyrrolidone solution ofpoly(vinylidene fluoride) as a binder was mixed therewith. This mixturewas applied to one side of a positive-electrode current collector 12comprising an aluminum foil and then dried. The foil coated was pressedso that the resultant positive composite 11 came to have a thickness of0.1 mm. The positive electrode 1 was obtained through these steps.

The negative electrode 2 was obtained in the following manner. First, agraphite as a negative-electrode material was mixed with anN-methyl-2-pyrrolidone solution of poly(vinylidene fluoride) as abinder. This mixture was applied to one side of a negative-electrodecurrent collector 22 comprising a copper foil and then dried. The foilcoated was pressed so that the resultant negative composite 21 came tohave a thickness of 0.1 mm. The negative electrode 2 was obtainedthrough these steps.

As the separator 3 was used a microporous film made of polyethylene(thickness, 25 μm; porosity, 50%).

The power-generating element 4 was produced by superposing the positiveelectrode 1, the separator 3, and the negative electrode 2 in this orderso that the positive composite 11 and the negative composite 21 facedeach other, with the separator 3 interposed between these.

Subsequently, the power-generating element 4 was immersed in thenonaqueous electrolyte to thereby infiltrate the nonaqueous electrolyteinto the power-generating element 4. Furthermore, the power-generatingelement 4 was covered with a metal/resin composite film 5 and the foursides thereof were sealed by thermal fusion bonding to therebyconstitute a sheath.

The nonaqueous electrolytes of Examples 1 to 4 and Comparative Examples1 and 2 were used to produce invention batteries 1 to 4 according to theinvention and comparative batteries 1 and 2, respectively. The designcapacity of each of these lithium batteries according to these Examplesis 10 mAh.

(Initial Charge/Discharge Test)

Subsequently, invention batteries 1 to 4 and comparative batteries 1 and2 described above were examined for first-charge capacity andfirst-discharge capacity. As first charge was conducted constant-currentconstant-voltage charge at 20° C. under the conditions of a current of 2mA and a final voltage of 4.2 V. The charge capacity thus obtained wastaken as the first-charge capacity. Subsequently to the first charge,constant-current discharge was conducted as first discharge at 20° C.under the conditions of a current of 2 mA and a final voltage of 2.7 V.The discharge capacity thus obtained was taken as the first-dischargecapacity. With respect to each battery, the proportion of thefirst-discharge capacity to the first-charge capacity was determined interms of percentage, and this value was taken as the initial efficiency.

(High-Rate Discharge Test)

Subsequently, invention batteries 1 to 4 and comparative batteries 1 and2 were subjected to a high-rate discharge test.

The test temperature was 20° C. As charge was conducted constant-currentconstant-voltage charge under the conditions of a current of 2 mA and afinal voltage of 4.2 V. As discharge was conducted constant-currentdischarge under the conditions of a current of 5 mA and a final voltageof 2.7 V. The battery capacity thus obtained was taken as the high-ratedischarge capacity.

The results obtained are shown in Table 1 and FIG. 2.

TABLE 1 Concentration First- First- High-rate of quarternary chargedischarge Initial discharge ammonium capacity capacity Efficiencycapacity salt mol/L mAh mAh % mAh Invention 0.06 11.5 9.5 82.6 5.8battery 1 Invention 0.1 11.2 9.8 87.5 6.0 battery 2 Invention 0.3 10.99.9 90.8 5.8 battery 3 Invention 0.5 11.0 9.3 84.5 4.4 battery 4 Com-0.7 11.3 8.5 75.2 2.5 parative battery 1 Com- 0 21.1 1.7 7.9 0.8parative battery 2

As apparent from Table 1 and FIG. 2, comparative battery 2, whichcontained no quaternary ammonium salt, had an exceedingly largefirst-charge capacity, which was about 2 times the design capacity, andhad an exceedingly small first-discharge capacity. Namely, thiscomparative battery had an exceedingly low initial efficiency. Thereason for these results is thought to be that the propylene carbonatein the nonaqueous electrolyte underwent decomposition on the graphitenegative electrode during first charge and this resulted in a reducedreversible capacity.

In contrast, invention batteries 1 to 4, in which a quaternary ammoniumsalt had been added in an amount of 0.06-0.5 mol/L, were excellent inboth first-charge capacity and first-discharge capacity and showed amarkedly improved initial efficiency. In addition, these batteriesshowed markedly improved high-rate discharge characteristics. Thereasons for these results are thought to be as follows. During firstcharge, the quaternary ammonium salt decomposed on the graphite negativeelectrode to form, on the surface of the graphite negative electrode, alithium ion-permeable protective coating film which was dense and hadexcellent permeability to lithium ions. This protective coating filmsurely inhibited the organic solvent used in the nonaqueous electrolytefrom decomposing.

On the other hand, comparative battery 1, in which a quaternary ammoniumsalt had been added in an amount of 0.7 mol/L, was relativelysatisfactory in first-charge capacity and first-discharge capacity.However, this battery had a considerably small high-rate dischargecapacity, which was about 30% of the first-discharge capacity. Althoughthe reason for this has not been fully elucidated, it is thought thatthe small high-rate discharge capacity is attributable to an increase inthe viscosity of the nonaqueous electrolyte or an increase in theresistance of the protective coating film.

In the Examples given above, nonaqueous electrolytes containing ethylenecarbonate and propylene carbonate were used. However, nonaqueouselectrolytes containing butylene carbonate, γ-butyrolactone, andγ-valerolactone were ascertained to likewise bring about the effects ofthe invention.

Especially in nonaqueous electrolytes containing either of propylenecarbonate and butylene carbonate, the effects of the invention wereparticularly markedly observed as in the Examples given above, incontrast to the corresponding system which contained no quaternaryammonium salt.

Incidentally, as FIG. 2 suggests, the influences of the content of aquaternary ammonium salt on initial efficiency and high-rate dischargecharacteristics were likewise observed regardless of the kind of thequaternary ammonium salt.

INDUSTRIAL APPLICABILITY

As described above, a nonaqueous-electrolyte battery which is highlysafe and has a high charge/discharge efficiency and a high energydensity can be easily provided according to the invention. The inventionhence has a high industrial value.

1. A nonaqueous electrolyte, comprising: an organic solvent and alithium salt dissolved in the organic solvent; and a quaternary ammoniumsalt in an amount of 0.06 mol/L or greater and 0.5 mol/L or less, thequaternary ammonium salt having a structure represented by (chemicalformula 2):

(wherein R is a divalent organic linking group having a main chain whichhas 4-5 atoms and is constituted of at least one member selected fromcarbon, oxygen, nitrogen, sulfur, and phosphorus; R1 and R2 each areeither an alkyl group having 1-6 carbon atoms or an alkyl group in whichat least one of the hydrogen atoms has been replaced by a fluorine atom;and X⁻ is a fluorine-containing anion).
 2. The nonaqueous electrolyte ofclaim 1, wherein said organic solvent comprises one or more organicsolvents selected from the group consisting of ethylene carbonate,propylene carbonate, butylene carbonate, γ-butyrolactone, andγ-valerolactone.
 3. The nonaqueous electrolyte of claim 1, wherein thenonaqueous electrolyte comprises one or more members selected from thegroup consisting of BF₄ ⁻, PF₆ ⁻; CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, N(C₂F₅SO₂)₂ ⁻,N(CF₃SO₂)(C₄F₉SO₂)⁻, C(CF₃SO₂)₃ ⁻, and C(C₂F₅SO₂)₃ ⁻.
 4. Anonaqueous-electrolyte battery, comprising: a positive electrode, anegative electrode, and a nonaqueous electrolyte according to claim 1.5. The nonaqueous-electrolyte battery of claim 4, wherein the negativeelectrode comprises a graphite.
 6. The nonaqueous-electrolyte battery ofclaim 4, further comprising: a sheath formed over said positive andnegative electrodes and said electrolyte, said sheath comprising ametal/resin composite material.
 7. A nonaqueous-electrolyte batterywhich comprises a positive electrode, a negative electrode, and anonaqueous electrolyte according to claim
 2. 8. A nonaqueous-electrolytebattery which comprises a positive electrode, a negative electrode, anda nonaqueous electrolyte according to claim
 3. 9. The nonaqueouselectrolyte of claim 1, wherein said organic solvent comprises a memberselected from the group consisting of propylene carbonate and butylenecarbonate.
 10. The nonaqueous electrolyte of claim 1, wherein thequaternary ammonium salt having a structure represented by chemicalformula 2 comprises a combination of an anion and a member selected fromthe group consisting of a pyrrolidiniurn cation, piperidinium cation,and pyrrolium cation.
 11. The nonaqueous electrolyte of claim 10,wherein the pyrrolidinium cation comprises a member selected from thegroup consisting of a 1,1-dimethylpyrrolidinium ion,1-ethyl-1-methyl-pyrrolidinium ion, 1-methyl-1-propylpyrrolidinium ion,and 1-butyl-1-methylpyrrolidinium ion, wherein the piperidinium cationcomprises a member selected from the group consisting of a1,1-dimethylpiperidinium ion, 1-ethyl-1-methylpiperidinium ion,1-methyl-1-propylpiperidinium ion, and 1-butyl-1-methylpiperidinium ion,and wherein the pyrrolium cation comprises a member selected from thegroup consisting of a 1,1-dimethylpyrrolium ion,1-ethyl-1-methylpyrrolium ion, 1-methyl-1-propylpyrrolium ion, and1-butyl-1-methylpyrrolium ion.
 12. The nonaqueous electrolyte of claim1, wherein said amount of said quaternary ammonium salt is 0.1 mol/L orgreater and 0.35 mol/L or less.
 13. The nonaqueous electrolyte of claim1, wherein said lithium salt comprises a member selected from the groupconsisting of LiBF₄, LiPF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, and LiC(C₂F₅SO₂)₃.
 14. Anonaqueous-electrolyte battery, comprising: a power generating unitcomprising a positive electrode, a negative electrode, and a separatorinterposed between said positive and negative electrodes; and anonaqueous electrolyte impregnated into said power generating unit, saidnonaqueous electrolyte comprising: an organic solvent and a lithium saltdissolved in the organic solvent; and a quaternary ammonium salt in anamount of 0.06 mol/L or greater and 0.5 mol/L or less, the quaternaryammonium salt having a structure represented by (chemical formula 2):

(wherein R is a divalent organic linking group having a main chain whichhas 4-5 atoms and is constituted of at least one member selected fromcarbon, oxygen, nitrogen, sulfur, and phosphorus; R1 and R2 each areeither an alkyl group having 1-6 carbon atoms or an alkyl group in whichat least one of the hydrogen atoms has been replaced by a fluorine atom;and X⁻ is a fluorine-containing anion).